Conventionally, as a film covered battery which uses a thermally sealable film as a casing material, there is known one which has a structure in which a cell element is covered with a laminate film, and open edges of the laminate film are thermally sealed, with lead ends of a positive pole and a negative pole connected to the cell element protruded from the laminate film, thereby hermetically sealing (hereinafter simply called “seal” in some cases) the cell element. The laminate film used herein is made up of a metal layer and a thermally sealable resin layer laminated to each other. This type of film covered battery has the advantage of being readily reduced in thickness, and most of conventional film covered batteries have flat shapes.
Like those using other casing materials, a battery which uses a film for a casing material is also required to ensure the sealing reliability in a sealed portion so as to prevent introduction of external air into the battery and leakage of an electrolytic solution in the battery. Particularly, for a battery which includes a nonaqueous electrolytic solution (hereinafter sometimes called the “nonaqueous electrolytic battery”), the sealing reliability is critical. Defective sealing, if any, would cause the electrolytic solution to deteriorate due to components of external air, so that the battery performance significantly deteriorates.
Also, in a battery which uses a film for a casing material, particularly when a cell element has a certain degree of thickness, the film is generally deep-draw molded in the shape of a cup in conformity to the outside shape of the cell. This is intended to improve a cell element containment efficiency and to improve the outer appearance of the battery.
As such a conventional film covered battery, JP-A-2000-133216 discloses a battery in which a casing material made of an aluminum laminate film is formed with a cup area conformal to the outside shape of a cell element through deep-draw molding, and the casing materials are thermally sealed near the cell element. According to this document, by forming the casing material with the cup area conformal to the outside shape of the cell element, the occurrence of wrinkles is prevented even if the casing materials are thermally sealed to each other quite near the cell element for sealing, thus providing a volumetrically efficient battery.